Video processing systems are known in which video signals representing a television picture are processed in real time to produce effects in which the picture appears to be manipulated in three dimensional space. An example of such a system previously manufactured by the present Assignee (Applicant) enabled the production of effects such as a three dimensional spin of the picture or a change in the position, size or view position of the picture using manual controls including a tracker ball. To produce these effects a frame of video signals to be processed is written to a framestore and a processing circuit responsive to signals set up by the manual controls determines successive addresses in the framestore from which successive video signals should be read in order to build up an output frame containing the processed picture. A monitor is provided for displaying successive processed frames as a moving picture so that an operator can observe the effect of his manipulation of the controls in real time. During each frame period the processing circuit responds to incremental control signals set up by use of the manual controls during the preceding frame.
One use to which a system such as that described hereinabove can be put is that of transforming a flat picture so that it can be placed exactly over a polygon (usually quadrilateral) on another picture to build up a composite picture. Such a transformation may be required for example where the first picture is say a view through a window and it is desired to place the first picture over a window in the second picture seen from different positions. While it is possible to achieve this result using the above described system there are practical problems because realistic superimposing of the first picture will involve a combination of three dimensional spin and changes in position, size and point of view of the first picture.
Another video processing system currently manufactured by the present Assignee (Applicant) and sold under the trade name "ENCORE" is described in our European Patent Application, Publication No. 0,283,159 and corresponding patent applications in the United States of America (Serial No. 159,780) now U.S. Pat. No. 4,951,040 and Japan (Serial No. 162099), the teachings of which are incorporated herein by reference. In the image processing systems described in these patent applications means are provided for representing the addresses of at least four reference points defining corners of a polygon notionally projected onto a viewing screen. User operable control input means enable the user to produce selected adjustment of the addresses to cause the reference points to define the corners of the polygon projected onto the screen after movement of the polygon in three dimensional space, and transforming means transform the addresses of individual picture elements which make up a video picture so as to cause the video picture to appear to be projected onto the viewing screen after undergoing the same movement in three dimensional space as the polygon. Thus, in this image processing system a three dimensional translation of a video picture can be defined by moving the corners of an originally rectangular polygon representing corners of an originally rectangular image to the corners of a quadrilateral representing the image screen in perspective. This technique is known as "corner pinning" and makes it possible to pin a video image over say a window defined in another video image to produce a combined video image.
Whilst this system provides satisfactory results in combining two images when the position of the insert picture remains substantially fixed, difficulties are encountered when the position and shape of the insert picture is required to change from one frame to the next in the other video picture. This is because the user is free to define the shape and movement of the polygon in three dimensional space but is unable to relate these parameters with particular frames in the two pictures. As a result unwanted artefacts, in the form of a visible boundary between the insert picture and the other picture or flicker for example, may appear in the combined video picture. In such cases the combined picture will be unacceptable.